The events surrounding cell activation, proliferation, and differentiation are just beginning to be delineated. We have identified a novel cell surface antigen (53.6 Ag) whose expression is associated with cell activation and proliferation. The studies presented in this application are intended to define the role of this molecule in these processes. This will be accomplished by using two major approaches: first, the antigen and its regulation will be characterized at the nucleic acid level, by isolation and subsequent use of the gene encoding the antigen. Second, the function of the antigen in cellular processes will be analyzed, both by studying its relationship to other molecules which function in cell activation, and by disruption of antigen expression and examination of the consequences. A cDNA clone encoding the 53.6 Ag will be isolated after purification and partial amino acid sequencing of the protein, using oligonucleotide screening of cDNA libraries. Two experimental systems will be used for studies of the regulation of the 53.6 Ag: Mitogen-stimulated lymphocytes, and transformed cell lines which differentiate in response to chemical inducers. Peripheral blood lymphocytes are a normal population of quiescent cells, of which a large proportion can be stimulated by mitogens. Cell lines which differentiate in response to chemical inducers offer large numbers of relatively homogeneous cells which can give rise to approximately normal terminal progeny. Both of these systems can be manipulated to alter the cellular response to stimulation; thus, the expression of the 53.6 Ag can be observed in the same cells under different circumstances. The analysis of the expression and regulation of the 53.6 Ag in these systems is expected to provide new insights on the events which occur in cell activation and differentiation. Because one of the salient characteristics of cellular transformation and tumorigenesis is the acquisition of the ability to proliferate indefinitely or inappropriately, the results of these experiments will also have implications for the mechanisms of transformation.